A spark plug includes a center electrode disposed along the axis thereof and a ground electrode disposed with a gap formed between the ground electrode and a forward end portion of the center electrode, and ignites an air-fuel mixture introduced into a combustion chamber of an internal combustion engine, etc., through generation of spark discharges between the electrodes. Since electrodes used in a spark plug have concern for not only erosion stemming from spark discharges, but also erosion stemming from oxidation or the like caused by exposure to combustion gas, electrode materials having excellent durability have conventionally been developed (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2002-260818).
Meanwhile, there exists a spark plug in which, in order to cope with erosion of electrodes stemming from spark discharges, noble metal tips are joined to respective end portions of the electrodes between which spark discharges are generated, thereby exhibiting excellent resistance to spark-induced erosion (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2003-197347). Furthermore, there exists a spark plug in which a noble metal tip is joined to an end of an electrode and in which a relatively small size is imparted to the noble metal tip for improving ignition performance (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2002-313524).
Incidentally, in association with tendency toward higher outputs, etc., of engines, the environment in which spark plugs are used is becoming more severe. Accordingly, further improvement of durability is required of electrodes of spark plugs. Conventionally, in order to meet the requirement, NCF600, NCF601, etc., have been used as electrode materials.
Upon exposure to a high-temperature atmosphere, an Ni alloy which contains Al, such as INCONEL (registered trademark) 601, forms an Al oxide layer on its surface, thereby restraining oxidation-induced erosion of an electrode material and thus securing resistance to high-temperature oxidation. However, the following has been found: since Al is highly reactive with nitrogen, Al and nitrogen react with each other to deposit Al nitride. As a result, Al nitride is formed as lumps in a region located internally of the Al oxide layer. Al nitride is hard, and a region dotted with Al nitride is embrittled. The higher the temperature and the longer the high-temperature retention, the more deeply such Al nitride deposits in the electrode material. Accordingly, in the case of a thin electrode material, Al nitride may deposit across the entire thickness.
Also, the following has been found: when an electrode to which a noble metal tip is joined is exposed to a high-temperature atmosphere, electrode material components partially diffuse into the noble metal tip and react with a noble metal, thereby forming a low-melting-point compound. Formation of such a low-melting-point compound leads to deterioration in resistance to spark-induced erosion and resistance to oxidation of the noble metal tip, and leads further to deterioration in reliability of joining the noble metal tip to the electrode.